Electron plane reflection
This workflow will work for your filled circle and an reflexion angle equal to the incident angle.

Use Mirror Modifier

To create the reflection we can create a double of the electrons with the mirror modifier.

You need to set the origin of the electrons object on the surface of the reflecting face: 

Select the reflecting face and set the 3d cursor to its surface with shift+s -> Cursor to Selected, then go in Object mode, select the electrons and press Ctrl+Alt+Shift+C -> Origin to 3D cursor.

Now you can add a mirror modifier to your electrons and select the correct Axis to have your electrons mirrored under the reflexion Plane

Animate with ShapeKey

We can't animate the object with normal keyframe since we don't want to modifie the origin point set on the reflexion plane.

You can do an animation with shapekey.

Create a first shapekey of the electrons far from the reflexion plane. And create a second shapekey by moving your circle with 3D manipulator and 'Normal' Selected as transform orientation.

Like so your electrons will move with the correct orientation. Already here if you sweep your shape key value, the animation is correct but you still have the mirrored electrons under the reflexion plane.

Hide the Electrons under the plane

To hide the Electron I created a big cube with some subdivision that incorporate electrons under the reflexions plane. Be sure to make it invisible to your render.

To your Electrons, add a Boolean Modifier, set the Operation to Difference and the Object to your newly created Cube. And voilà your electrons are now only under the reflexion plane. 

Result:



Here is the blend file I created if you want to play with.